The invention relates to a method for controlling the power source of a welding, cutting or plasma coating system, wherein the power source comprises a measuring part and a controllable clocked power part and wherein a measure value, which represents a measure of a characteristic variable, that is to be controlled, of the power part, is determined by means of the measuring part, and the measure value is used for controlling the power part.
Electric welding systems as well as electrical cutting systems and plasma coating systems have a power source with a power part, which provides a controllable output voltage or a controllable output current or makes it possible for the output current and the output voltage to undergo combined control. In the case of electric welding systems, a welding electrode may be connected to the power part by way of an electric cable to form an arc between the welding electrode and a workpiece to be welded. In a corresponding way, in the case of plasma coating systems, a controllable output voltage or a controllable output current may be provided by means of the clocked power part, so that a plasma for coating a workpiece can be produced within a coating chamber. For it to be supplied with energy, the clocked power part may be connected to a supply network. In many cases, the power part has a transformer and electrical converters. Furthermore, in many cases the power part comprises electrical components for smoothing the output current and/or the output voltage. Used for this purpose, for example, are internal impedances, in particular coils and resistors. The electric cable that can be connected to the power part as well as the welding electrode and the arc or the plasma electrode and the electrical discharge in the coating chamber form external impedances and external ohmic resistances. It is desirable for the voltage drop across the arc or the electrical discharge to be detected as quickly and exactly as possible for purposes of process control. This voltage can be calculated from the output voltage of the clocked power part with allowance made for the external impedances and ohmic resistances.
The output voltage, the output current or else the emitted power of the clocked power part represent characteristic variables of the power part, which are usually controlled by means of a control part, for example time-dependently or as a reaction to specific process states. A measure value is often determined with the aid of a measuring part as a measure of the characteristic variable to be controlled, and is then used for controlling the power part. A mean value over time of the characteristic variable to be controlled may be determined, for example, as the measure value.
The characteristic variable is usually subject to variations over time on account of changes of the external circuit connected to the power part. In the case of electric welding systems, for example, the arc voltage may strongly fluctuate process-dependently on account of changes in the length of the arc and on account of the occurrence of short-circuits between the welding electrode and the workpiece to be welded, or else on account of breaking of the arc. These fluctuations in turn influence the output voltage of the clocked power part, which is also distinguished by periodic changes on account of the clocked operation of the power part.
For effective control of a characteristic variable of the power part, it is desirable to determine the measure value within a short time, in order also to allow short-time fluctuations of the characteristic variable to be counteracted. Potential-affected converters or else potential-isolating converters are usually used for determining the measure value. It is also known to detect the characteristic variable itself by means of the measuring part and to smooth it by way of lowpass elements or active filters of a higher order and to use the measured value smoothed in this way for controlling the characteristic variable. There are also known power sources in which the measured value already smoothed by way of lowpass elements is additionally subjected to an analog-digital conversion and then processed by means of a digital signal processor. These known methods have the disadvantage that they are relatively time-consuming, so that, in particular, short-term fluctuations of the characteristic variable of the power part that is to be controlled can only be insufficiently counteracted.
It is object of the present invention to develop a method of the type mentioned at the beginning in such a way that a measure of the characteristic variable to be controlled can be determined within a short time.